Conventional fuel injection systems employ a "jerk" type fuel system for pressurizing and injecting fuel into the cylinder of a diesel engine. A pumping element is actuated by an engine-driven cam to pressurize fuel to a sufficiently high pressure to unseat a pressure-actuated injection valve in the fuel injection nozzle. In one form of such a fuel system having an electromagnetic unit injector, the plunger is actuated by an engine driven cam to pressurize the fuel inside the bushing chamber when a solenoid is energized and the solenoid valve is closed. The metering and timing is achieved by a signal from an electronic control module (ECM) having a controlled beginning and a controlled pulse. In another form of such a fuel system, the fuel is pressurized by an electronic or mechanical pumping assembly into a common rail and distributed to electromagnetic nozzles, which inject pressurized fuel into the engine cylinders. Both the electronic pump and the electromagnetic nozzles are controlled by the ECM signal.
One problem with using a common rail results from the high pressures experienced in diesel engines, which are in the neighborhood of up to a maximum of 30,000 psi. Another problem in conventional fuel injection systems is achieving a controlled duration and cut-off of the fuel injection pressure. Standard fuel injection systems commonly have an injection pressure versus time curve (the fuel injection event profile) in which the pressure increases to a maximum and then decreases, following a somewhat skewed, triangularly-shaped curve. Such a pressure versus time relationship initially delivers a relatively poor, atomized fuel penetration into the engine cylinder because of the low injection pressure. When the pressure curve reaches a certain level, the pressure provides good atomization and good penetration. As the pressure is reduced from its peak pressure, the decreasing pressure again provides poor atomization and penetration, and the engine discharges high emissions of particulates and smoke.
One of the objects of fuel injection designers is to reduce unburned fuel by providing a pressure versus time curve having a square configuration, with an initially high pressure increase to an optimum pressure, providing good atomization, and a final sharp drop to reduce the duration of poor atomization and poor penetration.
Additionally, the optimum delivery of fuel to an engine cylinder (i.e. the profile of the injection curve) is dependent upon engine speed. Consequently, an injection pressure vs. time curve which is ideal at a first engine speed will be less than ideal at a second engine speed. Consequently, prior art fuel injectors have been designed to have a pressure vs. time curve which provides acceptable (but not optimum) performance at all engine speeds. There is therefore a need for a fuel injector which is capable of "rate shaping", i.e. changing the shape of its injection profile with changing engine speed. Such rate shaping allows for reduced emission of particulates and hydrocarbons and also reduced fuel consumption.
The present invention is therefore directed toward providing a high pressure electronically controlled common rail fuel injector which allows for rate shaping of the injection curve under the control of the engine ECM.